The present invention relates to level-shifters and, more particularly, to high voltage level-shifters utilized in integrated circuits that utilized both positive and negative operating voltages.
Digital devices typically employ voltage-sensitive binary logic. In binary logic devices, one voltage level represents a logic xe2x80x9c0xe2x80x9d or xe2x80x9clowxe2x80x9d while a different voltage level represents a logic xe2x80x9c1xe2x80x9d or xe2x80x9chighxe2x80x9d. In typical positive logic, the lower voltage level (e.g., ground) represents the logic xe2x80x9c0xe2x80x9d and the higher voltage level (e.g., 3.3 Volts) represents a logic xe2x80x9c1xe2x80x9d. In negative logic, the reverse is true.
A problem arises when digital devices within an electronic system often have different operating voltages (i.e., different voltage levels representing logic xe2x80x9c1xe2x80x9d). Variations in fabrication techniques have lead to digital devices using ever-decreasing operating voltages, and hence ever changing logic xe2x80x9c1xe2x80x9d voltage levels. Accordingly, when several digital devices are combined to form an electronic system, one device may use 5 Volts to represent a logic xe2x80x9c1xe2x80x9d, while another device may use 3.3 Volts. These differences in operating voltages create communication problems because the logic xe2x80x9c1xe2x80x9d signal output from one digital device may have too low of a voltage level to be interpreted as a logic xe2x80x9c1xe2x80x9d by another digital device in the system.
Level-shifters are circuits that facilitate communication between various digital devices by xe2x80x9cshiftingxe2x80x9d the logic xe2x80x9c1xe2x80x9d voltage levels output from one digital device to the voltage levels used by another device. A level-shifter that is incorporated in a digital device typically includes an output driver whose positive and negative supply rails are coupled to voltage supplies maintained at the operating voltage levels used in the digital device. Signals applied to the input terminal of the output driver are used to connect either the positive supply rail or the negative supply rail to its output terminal, thereby xe2x80x9cshiftingxe2x80x9d input signals to the voltage levels used in the digital device.
In addition to inter-device communications, high voltage level-shifters may be used in a device to switch between high voltage levels needed, for example, to program and erase electrically erasable programmable floating gate memory devices. In these applications, high voltage level shifters typically operate in essentially the same manner as that described above to shift low voltage data signals to high voltage program/erase signals. For example, during programming, data values are transmitted within the device using typical positive logic signals (e.g., 3.3 Volts represents logic xe2x80x9c1xe2x80x9d, and 0 Volts represents logic xe2x80x9c0xe2x80x9d), and are then shifted to the appropriate programming voltages (e.g., 5 Volts or 10 Volts), depending upon the data value stored in the flash memory cells.
A problem with prior art level-shifters arises when both output signal voltages are non-positive (i.e., either both below ground, or one at ground and the other below ground). For example, both positive and negative voltages are used to program some electrically erasable programmable floating gate memory devices to reduce the stress on chip elements by minimizing voltage magnitudes while maximizing the voltage potential used to program/erase the flash memory cells. During programming, data values are typically supplied using typical positive logic signals (e.g., 3.3 Volts represents logic xe2x80x9c1xe2x80x9d, and 0 Volts represents logic xe2x80x9c0xe2x80x9d), and then must be converted to the appropriate negative voltage (e.g., either xe2x88x922 Volts or xe2x88x928 Volts), depending upon the data value stored in the flash memory cells. The problem arises because conventional level-shifters, which utilize one positive voltage and ground, are not able to convert a positive voltage input signal to a negative voltage output signal.
What is needed is a high voltage level shifter that is able to convert positive voltage input signals negative voltage output signals.
The present invention provides a high voltage level-shifter that facilitates the conversion of typical positive voltage input signals to non-positive voltage (i.e., at or below ground) output signals. The high voltage level-shifter is separated into two parts referred to herein as xe2x80x9cvoltage shifting circuitsxe2x80x9d that are connected in parallel between a circuit input terminal and a circuit output terminal. The first voltage shifting circuit includes an output driver that transmits a first negative voltage signal on the circuit output terminal when the circuit input terminal receives an input signal having a first voltage level (e.g., logic xe2x80x9c1xe2x80x9d). Similarly, the second voltage shifting circuit transmits a second negative voltage (or ground) signal on the circuit output terminal when the input signal has a second voltage level (e.g., logic xe2x80x9c0xe2x80x9d).
In accordance with the present invention, the first voltage shifting circuit includes an isolation switch (e.g., a pass transistor) connected between a positive voltage source and a positive supply rail of the output driver. The negative supply rail of the output driver is connected to the first negative voltage source. An output control circuit controls the isolation switch to couple the positive voltage source to the positive supply rail of the output driver only when the negative voltage source is driven onto the circuit output terminal (e.g., in response to a logic xe2x80x9c1xe2x80x9d input signal). Conversely, when the circuit input terminal receives a logic xe2x80x9c0xe2x80x9d input signal, the output control circuit opens the isolation switch to decouple the positive voltage source from the positive supply rail of the output driver, thereby causing the positive supply rail to float. In addition, the output control circuit biases the output driver to couple the circuit output terminal to the floating positive supply rail, thereby effectively xe2x80x9ctri-statingxe2x80x9d the output driver. At the same time, the second voltage shifting circuit connects the circuit output terminal to the second negative voltage source (or ground). By decoupling the positive supply rail of the output driver in the first voltage shifting circuit, current flow from the positive voltage source to the second negative voltage source (or ground) is prevented.